chore() Remove more pieces of Opera < 15 support and document it

[realityking]

The test for the sniffer was already removed in 8202c4d

[tbosch]

Do you have a link that Operate 15+ does no more use the -O css prefix for css transitions / animations?

[realityking]

Nothing authoritative, but a quick look in the Opera Dev Console tells as much.

According to caniuse both properties were unprefixed in Opera 12.1 but Opera 15 reintroduced the prefix as -webkit- for animation.

[caitp]

the amusing thing is that we claim to support opera and yet don't test against it whatsoever --- it's more of a "maybe it works maybe it doesn't, who knows" kinda thing u_u but the patch looks good to me

[realityking]

Given the fact that Opera >= 15 is essentially Chrome, I personally don't test with it (automatically or manually) until and unless I receiver a bug report.

So far I had one this year, and I was able to trace that back to some unexpected settings that could have occurred in Chrome as well.